This invention relates to the three-dimensional microfabrication of structures and more particularly to the microfabrication of electrodeposited structures.
Making three-dimensional (3-D) micromachined objects is difficult using current techniques; there are few alternatives to using a large number process steps and photolithographic masks. An example is the formation of poly (dimethyl siloxane) (PDMS) replicas of micromachined master patterns for soft lithography and for the creation of microfluidic devices. See, A. Kumar, N. L. Abbott, H. A. Biebuyck, E. Kim, G. M. Whitesides, Acc. Chem Res. 1995, 28, 219; E. Kim, Y. Xia, G. M. Whitesides, J. Am. Chem. Soc. 1996, 118, 5722; and D. C. Duffy, J. C. McDonald, O. J. A. Schueller, G. M. Whitesides, Anal. Chem. 1998, 70, 4974. The fabrication of these microfluidic devices relies on bulk micromachining of silicon wafers or a thick layer of patterned photoresist to generate a master pattern—the reversed sense of the pattern is generated by casting a soft replica over this master. There is much interest in producing 3D structures using surface or bulk micromachining techniques for the creation of more complex masters for microfluidic devices, as well as for other uses. See, B. -H. Jo, D. J. Beebe, Proc. SPIE—Int. Soc. Opt. Eng. 1999, 3877, 222; O. Hoffman, G. Voirin, P. Niedermann, A. Manz, Anal. Chem. 2002, 74, 5243; and L. Griscom, P. Degenaar, B. LePioufle, E. Tamiya, H. Fujita, Jpn. J. Appl. Phys. Part 1 2001, 40, 5485. Much of the published work that describes 3D microfluidic devices relies on the layer-by-layer construction of the structures that requires a large number of masks and process steps and the awkward alignment and assembly of the individual layers. The master pattern shown in FIG. 1 demonstrates how easily a 3D microfluidic master can be formed, in this case, a gold film patterned onto a silicon nitride covered wafer. The technique can create devices with a large ratio between the thickest and thinnest structures (a ratio of 50:1 has been fabricated).
Pyrrole films have been extensively electrodeposited for more than 20 years and various dopants have been used. A. F. Diaz, K. K. Kanazawa, G. P. Gardini, J. Chem. Soc. Chem. Commun. 1979, 635; G. Sabouraud, S. Sadki, N. Brodie, Chem Soc. Rev. 2000, 29, 283. Of special interest for this technique are dopants that are highly chemically resistant since microfabrication often involves aggressive cleaning and etching steps. Although the exact mechanism for electrodeposition of polypyrrole (PPy) is not fully understood, it involves the oxidation of the pyrrole monomer followed by several chemical and electron-transfer reactions. G. Sabouraud, S. Sadki, N. Brodie, Chem Soc. Rev. 2000, 29, 283. Pyrrole electropolymerization propagates with a 3D nucleation/growth pattern under charge transfer control. Films grow with different time constants in the upward and lateral directions. M. E. G. Lyons, Adv. Chem. Phys. 1996, 94, 297. The time constants vary as a function of current density, temperature, dopants, etc.
It is also known to use electroforming and molding to prepare planar microdevices with critical features of tens to thousands of micrometers. Such a technique uses metal electroplated into an x-ray defined poly(methyl methacrylate) (PMMA) mold. This prior art technique is known as LIGA from its German language acronym for making microelectromechanical systems (MEMS). E. W. Becker, W. Ehrfeld, P. Hagmann, A. Maner, D. Muenchmeyer, Microelectron. Eng. 1986, 4, 35. Many LIGA MEMS are produced from nickel.
An object of the invention therefore is a simpler technique for microfabrication of structures using very few steps and a single photolithographic mask.